1. Field of the Invention
The present invention relates to a radiation image detecting device having an emission start judging function that judges a start of radiation emission, and an operating method of the radiation image detecting device.
2. Description Related to the Prior Art
In a medical field, an X-ray imaging system using X-rays, as a kind of radiation, is known. The X-ray imaging system is constituted of an X-ray generating apparatus for generating the X-rays, and an X-ray imaging apparatus for taking an X-ray image of an object (patient) by receiving the X-rays passed through the object. The X-ray generating apparatus includes an X-ray source for emitting the X-rays to the object, a source control unit for controlling the operation of the X-ray source, and an emission switch for commanding the source control unit to start X-ray emission from the X-ray source. The X-ray imaging apparatus includes an X-ray image detecting device for detecting the X-ray image based on the X-rays passed through the object, and a console that controls the operation of the X-ray image detecting device and stores and displays the X-ray image.
The X-ray image detecting device has an image detector e.g. a flat panel detector (FPD) for detecting the X-ray image as an electric signal, and an imaging stand or a portable housing loaded with the FPD. The X-ray image detecting device that has the image detector contained in the portable housing is called an electronic cassette. The image detector has a panel unit having a two dimensional array of pixels each of which accumulates signal charge in response to the X-rays, and a control unit for controlling the operation of the panel unit. This control unit carries out an accumulation operation for accumulating the signal charge in the pixels, and an image readout operation for reading out an image signal corresponding to the signal charge of each pixel on a row-by-row basis of the pixels.
Such an X-ray image detecting device performs synchronization control for synchronization between a start of X-ray emission from the X-ray source and a start of the accumulation operation, so as to carry out the accumulation operation in synchronization with the X-ray emission. The synchronization control can be performed based on a judgment on the start of the X-ray emission from the X-ray source, instead of using a synchronization signal from the X-ray generating apparatus. In this case, the X-ray image detecting device has the function of judging the start of the X-ray emission (refer to US Patent Application Publication No. 2011/0180717 corresponding to Japanese Patent Laid-Open Publication No. 2011-174908, U.S. Pat. Nos. 8,507,871 and 8,629,406 corresponding to Japanese Patent Laid-Open Publication No. 2012-075077, US Patent Application Publication No. 2013/0037699 corresponding to Japanese Patent Laid-Open Publication No. 2011-223508, and U.S. Pat. No. 8,476,597 corresponding to Japanese Patent Laid-Open Publication No. 2012-110565).
The X-ray image detecting device having the emission start judgment function is provided with an X-ray detector for detecting the X-rays, a dose sampling unit for periodically sampling a dose signal that represents an X-ray dose (X-ray intensity) per unit of time based on output of the X-ray detector, and an emission start judgment unit that compares the dose signal with a predetermined threshold value and judges that the X-ray emission has been started when a signal value of the dose signal exceeds the threshold value. The dose sampling unit integrates the output of the X-ray detector in accordance with a sampling cycle and samples the integrated value as the dose signal. According to the US Patent Application Publication No. 2011/0180717 and the U.S. Pat. Nos. 8,507,871, 8,629,406, and 8,476,597, a part of the pixels in the panel unit are used as the X-ray detectors.
The emission start judgment requires high responsivity in order to minimize waste of the X-rays applied to the object. This is because the amount of wasted exposure of the object grows with the amount of time from the start of the X-ray emission to the start of the accumulation operation, since the X-rays applied during that time is not reflected in formation of the X-ray image. Therefore, for the purpose of speedup of the emission start judgment, the US Patent Application Publication No. 2011/0180717 and the U.S. Pat. Nos. 8,507,871 and 8,629,406 propose to set the sampling cycle of the dose signals by the dose sampling unit shorter than a readout period of the image signals of one row. The short sampling cycle shortens sampling intervals of the dose signals, so it is possible to obtain the dose signals early after the X-rays have been actually emitted. As a result, the start of the X-ray emission can be judged quickly.
On the other hand, various types of noise such as stationary noise occurring stationarily and vibration noise caused by shock or vibration from the outside occur in an electric circuit including the dose sampling unit. The noise is applied to the dose signal, and in some cases, brings about a malfunction in the emission start judgment, in such a situation that the dose signal exceeds the threshold value though no X-ray is applied in actual fact. To prevent such a malfunction, the X-ray image detecting devices according to the US Patent Application Publication No. 2013/0037699 and the U.S. Pat. No. 8,476,597 perform the emission start judgment in two steps, that is, a first judgment process and a second judgment process.
Both of the first judgment process and the second judgment process are performed based on the dose signal sampled by the dose sampling unit at the common sampling cycle. In the first judgment process, it is judged whether or not the dose signal is more than the threshold value. In a case where it is judged in the first judgment process that the dose signal is more than the threshold value, the second judgment process is carried out to judge whether or not a result of the first judgment is correct. The second judgment process checks variation in the dose signal with time. If the dose signal keeps having a value more than the threshold value for a predetermined duration, the result of the first judgment is judged to be correct.
As described above, the US Patent Application Publication No. 2013/0037699 and the U.S. Pat. No. 8,476,597 carry out the two-step judgment to improve precision. However, for further improvement of the precision, it is important to make a judgment based on the dose signal having a high S/N. Since the amount of the stationary noise of the dose sampling unit hardly varies with increase or decrease in the X-ray dose, the S/N of the dose signal is increased with increase in the signal value in accordance with the X-ray dose. To increase the signal value, the sampling cycle, which corresponds to a period of integrating the output of the X-ray detector, is preferred to be long. On the other hand, in order to improve the responsivity, the sampling cycle is preferred to be short.
Exposure control of the object tends to become strict more and more in recent years. Considering such circumstances, the X-ray imaging system is on its way to shorten the emission time and lower the X-ray dose per unit of time. In the emission start judgment, the shorter the emission time, the more severely the high responsivity is required. The lower the X-ray dose, the lower the S/N of the dose signal becomes, and hence the more importance it becomes to obtain the high precision than ever before. To meet the needs for the short emission time and the low X-ray dose, both of the high responsivity and the high precision are required.
However, the conventional X-ray image detecting device cannot satisfy the request for both of the high responsivity and the high precision, because improving one of the responsivity and the precision impairs the other.